The present invention relates to magnetic storage/read systems and more particularly to a magnetic storage/read system having a high recording density.
A prior art magnetic storage/read system in which metallic magnetic and nonmagnetic films having different magnetic properties are stacked is disclosed in Journal of the Physical Society of Japan, Vol. 59, No. 9, September 1990, pp. 3061-3064.
Also disclosed in Physical Review B, Vol. 43, No. 1, Jan. 1, 1991, pp. 1297-1300, is another prior art magnetic storage/read system in which an FeMn antiferromagnetic film is laminated with a ferromagnetic film, a nonmagnetic film and a ferromagnetic film.
JP-A-2-61572 discloses a magnetic field sensor which uses a ferromagnetic thin film separated by an interlayer and an antiferromagnetic thin film adjacent to one side of the ferromagnetic thin film.
Further disclosed in JP-A-2-23681 is a multilayer film which comprises a ferromagnetic thin layer having magnetoresistive effects and a metallic conductive thin layer.
In the prior art techniques, it has been difficult to realize a magnetic storage/read system having a high recording density and particularly a function as a storage device comprising a magnetoresistive element which provides sufficient sensitivity and output in response to an applied field.
It is an object of the present invention to provide a magnetic storage/read system which has a high recording density.
In accordance with an aspect of the present invention, a magnetic storage/read system, which in use is connected to an external device such as a computer, comprises preferably a recording medium for magnetically storing a signal, an electric-magnetic transducer opposing the recording medium, means for rotating the recording medium relative to the electric-magnetic transducer, and means for moving the electric-magnetic transducer to a predetermined position relative to the recording medium. It is more desirable that the magnetic storage/read system further comprises an interface circuit through which the system is connected to an external data processor and a circuit for processing signals stored on the recording medium.
The recording medium used in the present invention refers to a so-called magnetic disk which has a magnetic film for magnetically storing signals thereon, a substrate and a protective layer. The signal to be recorded on the recording medium may be recorded longitudinally in or perpendicularly to the plane of the recording medium. The magnetic film of the recording medium is required to have a coercivity that can magnetically hold the signals thereon.
The electric-magnetic transducer used in the present invention refers to a so-called magnetic head which can record signals on the recording medium or reproduce signals recorded thereon. The magnetic head used in the present invention is a dual type head having separate write and read heads. In particular, the read head preferably comprises a magnetoresistive head which detects a leakage magnetic field from the recording medium and reproduces the detected field in the form of a change in an electric resistance.
The magnetoresistive head is also referred to as a magnetoresistive element. The magnetoresistive element used in the present invention preferably comprises a magnetoresistive film for detecting a leakage magnetic field from the recording medium, a domain control film for controlling a magnetic domain created in the magnetoresistive film, a shunt film for applying a magnetic field for prescribing the magnetic driving range of the magnetoresistive film in a specific direction, and a soft magnetic film. In this case, the shunt film refers to an electrically conductive film formed adjacent to the magnetoresistive film such as a tantalum film or a copper film, and acts to apply a magnetic field generated by a current flowing therethrough to the magnetoresistive film. Similarly, the soft magnetic film refers to a soft magnetic film separated from the magnetoresistive film by a nonmagnetic film, and acts to apply to the magnetoresistive film a bias field generated by static coupling at an edge of the magnetoresistive film. These films do not physically contribute to the magnetoresistive effect per se but merely control the magnetization direction of the magnetoresistive film.
In accordance with an aspect of a magnetic storage/read system of the present invention, a magnetoresistive element in the magnetic storage/read system is made of a substrate, an antiferromagnetic film, a magnetic film, a nonmagnetic conductive film, a soft magnetic film, a nonmagnetic conductive film, a magnetic film and an antiferromagnetic film which are sequentially layered on said substrate, and said magnetoresistive element is driven relative to said medium in the longitudinal direction of the recording medium.
The antiferromagnetic film is made preferably of nickel oxide, but may be made of iron-manganese alloy, chromium-manganese alloy or chromium-aluminum alloy, or may be made of a hard magnetic material such as cobalt-platinum alloy or iron-cobalt-terbium alloy. The hard magnetic film is assumed to have a property that it is difficult to change its magnetization with an applied field and to have a coercivity of, e.g., above 100 Oe. Then, even when 50 Oe of magnetic field is applied, the direction of the magnetization of the hard magnetic film will not be substantially changed. Therefore, the hard magnetic film has an effect similar to that of the antiferromagnetic film. That is, the hard magnetic film has a property that when it contacts another magnetic film, it can apply unidirectional anisotropy based on the exchange coupling bias effect. It is not necessary to use an antiferromagnetic film, and instead a film generally called a bias film may be used.
The ferromagnetic film is made preferably of an alloy of 70-95 atomic % of Ni, 5-30 atomic % of Fe and 1-5 atomic % of Co or made of an alloy of 30-85 atomic % of Co, 2-30 atomic % of Ni and 2-50 atomic % of Fe. The ferromagnetic film may be also made of Permalloy or Permendur alloy. In other words, any material that is ferromagnetic and has a good soft magnetic property can be preferably employed for the ferromagnetic film.
The nonmagnetic film is made preferably of any one selected from the group of Au, Ag and Cu. The film may be made of Cr, Pt, Pd, Ru, Rh or an alloy thereof. That is, any material that does not have a spontaneous magnetization property at room temperature and has a high electron transmission property is preferably used as the material of the nonmagnetic film. The film has a thickness of preferably about 2-1000 xc3x85.
Further, in place of the nonmagnetic conductive film, a very thin nonmagnetic insulating film may be used. In other words, this film is only required to allow electron movement between the magnetic films, such that an electron tunneling effect through a nonmagnetic insulating film may be employed if necessary. In the latter case, it is necessary to make the nonmagnetic insulating film thin enough to enable the electron tunneling effect. The nonmagnetic insulating film is formed to have a thickness of generally below 100 xc3x85 and preferably below 50 xc3x85. The nonmagnetic insulating film may be formed preferably as a surface oxide of the soft magnetic film or as an aluminum oxide layer on a metallic film such as an aluminum film additionally formed on the soft magnetic film. The nonmagnetic insulating film may be made of alumina as necessary. In other words, a film having a property of interrupting the magnetic coupling between the magnetic films is preferably used as the nonmagnetic insulating film.
The substrate is used as a base for formation of the above films and acts preferably as a slider for the magnetic disk. The material of the substrate is preferably a ceramic such as stabilized zirconia and alumina, or silicon.
The magnetoresistive element having such a film structure as mentioned above has a property that its electric resistance varies with a very weak magnetic field applied thereto and its resistance change ratio is as large as 4-10%. For this reason, the magnetic storage/read system of the present invention can have a function of directly digitizing a recorded analog signal in its reproduction or read mode and advantageously can have a high recording capacity per disk area, i.e., a high recording density.
Also, the film structure may comprise a flat alumina or nickel oxide film formed on the substrate or a film of iron, titanium, zirconium, hafnium, niobium or cobalt-iron alloy formed on the substrate as an underlayer.
That is, it is preferable that the film formed on the substrate has a function of making flat the multilayered film formed thereon and has a uniform and flat film structure on the substrate or base, and the metallic films have thicknesses of 20 to 200 xc3x85 and the nonmetallic films have thicknesses of about 5 to 1000 xc3x85.
In accordance with another aspect of the present invention, there is provided another example of the magnetoresistive element used in the magnetic storage/read system, which element has a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films and in which an antiferromagnetic film directly contacts one of the magnetic films while a hard magnetic film directly contacts the other magnetic film.
The sandwich structure having the nonmagnetic conductive film disposed between the magnetic films means that the magnetoresistive element has a film structure which comprises at least a magnetic film, a nonmagnetic conductive film and a magnetic film sequentially stacked.
The magnetoresistive element used in the present invention, which has such a sandwich structure as mentioned above, has a function of causing electron spin dependent scattering in the interfaces between the magnetic films and the nonmagnetic film and also advantageously causing the magnetoresistive effect between the two magnetic films. The magnetoresistive element also has a function of interrupting the magnetic coupling between the magnetic films and advantageously improving its sensitivity to the magnetic field applied to the magnetoresistive element.
Further, since one of the magnetic films in the sandwich structure directly contacts the antiferromagnetic film and the other magnetic film directly contacts the hard magnetic film, the element has a function of applying anisotropies of different directions and strengths to the respective magnetic films and advantageously prescribing the magnetization direction and mobility of each magnetic film.
As a result, the magnetic storage/read system of the present invention advantageously can always provide a specific output to a varying signal, have good reproducibility and reduce an error rate in its reproduction mode.
In the formation of the film structure, the antiferromagnetic film may be formed on the substrate prior to the formation of the hard magnetic film, or the hard magnetic film may be formed on the substrate prior to the formation of the antiferromagnetic film.
That is, it is desirable that the element has a film structure made up of an antiferromagnetic film, a magnetic film, a nonmagnetic conductive film, a magnetic film and a hard magnetic film, and that the antiferromagnetic and hard magnetic films have thicknesses of about 20 to 2000 xc3x85.
Furthermore, in the magnetoresistive element used in the present invention, it has been found that a bias film made of antiferromagnetic material is preferably formed directly contacting the magnetic film as a means for inducing anisotropy in a specific direction.
Preferably, this bias film is positioned closer to the substrate than the magnetic and nonmagnetic films, is made flat on its surface to enhance the flatness of the layered films and to provide an underlayer function, and the anisotropy causes the domain structure of the magnetic film to be changed to a single domain, thus suppressing noise generation.
For example, the film structure comprises first and second magnetic films stacked with a nonmagnetic conductive film disposed therebetween and a bias film formed directly contacting the first magnetic film.
In this way, when the magnetoresistive element comprises a layered structure exhibiting the magnetoresistive effect and at least one pair of electrodes electrically contacting the layered structure for measurement of its electric resistance, the element can function as a magnetic field sensor and can detect a signal on the recording medium at a high sensitivity.
It is particularly desirable to sequentially stack on a base a first bias film, a first magnetic film, a first nonmagnetic conductive film, a second magnetic film, a second nonmagnetic conductive film, a third magnetic film, a second bias film and electrodes. In this connection, the third magnetic film preferably has the same magnetic characteristics as the first magnetic film.
Control over the biasing direction of the bias films and the anisotropy direction of the magnetic films is carried out preferably by applying a suitable magnetic field during deposition of the layered structure of the element according to the deposition process. It is also desirable that the layered structure is subjected to an in-field heat treatment during or after the formation of the layered structure of the element.
With regard to the application of the magnetic field during the film formation, it is desirable to control the direction and intensity of the magnetic field according to the film formation process to thereby control the bias applying direction of the bias films and the uniaxial anisotropy of the second magnetic film.
Further, when it is desired to subject the layered structure to an in-field heat treatment during the film formation, it is preferable to control the anisotropy of the first and third magnetic films and the uniaxial anisotropy of the second magnetic film.
In the case of providing a hard magnetic film as one of the bias films, it is desirable to apply a magnetic field after fabrication of the element to thereby orient the magnetization of the hard magnetic film in a predetermined direction.
When the anisotropy is controlled by the above method, the performance of the element can stably be kept high.
The material of the bias films preferably has a high electric resistance and more specifically has a resistivity above 5xc3x9710xe2x88x924 (xcexa9xc2x7cm). This bias film material prevents reduction of the output of the element caused by a current leakage and also controls the flatness of the layered structure. When a nickel oxide (NiO) film which is substantially an insulator is used as the bias film material, the field sensitivity become as high as about 10 Oe. Therefore, there can be realized a layered structure which has a reliability as high as about 2-4 times that of the prior art.
The magnetoresistive element used in the magnetic storage/read system of the present invention has a layered structure comprising at least a magnetic film, a nonmagnetic film and a magnetic film, in which one of the magnetic films has a magnetization fixed in a direction substantially perpendicular to the recording medium in the film plane while the other magnetic film has a rotatable magnetization initially oriented in a direction parallel to the recording medium in the plane of the other magnetic film in the absence of an applied magnetic field.
The xe2x80x98direction substantially perpendicular to the recording mediumxe2x80x99 means a state wherein the films of the magnetoresistive element are opposed perpendicularly to the recording medium and parallel to a direction normal to the plane of the recording medium. The phrase xe2x80x98substantially perpendicularxe2x80x99 refers to a range of from xe2x88x9210 to +10 degrees around the perpendicular direction and preferably a range of from about xe2x88x925 to about +5 degrees. The xe2x80x98direction parallel to the recording medium in the plane of the other magnetic filmxe2x80x99 refers to a direction that is parallel to the plane of the recording medium and lies in the plane of the other magnetic film. In other words, when no field is applied, the magnetization of the other magnetic film is oriented in the above parallel direction and rotates from this direction when a field is applied.
Magnetization is a physical quantity which means the magnitude of a spontaneous magnetic vector of a ferromagnetic material itself, and in the present invention, it has a function of responding to an applied field. The fixing of the magnetization direction of one of the magnetic films and enabling the magnetization of the other magnetic film to rotate correspond to means for changing the angle between the magnetizations of the magnetic films with respect to the applied field and provides a high-frequency response to the applied field which is regular and has no hysteresis. Since the element of the present invention has such a response, the element can detect a signal magnetically recorded on the recording medium and convert it into an electric signal. Thus, a magnetic storage/read system using the element of the invention can advantageously have a low error rate and a high recording density and can provide analog-to-digital conversion.
As an example of the method for fixing the magnetization of one magnetic film and enabling the magnetization of the other magnetic film to rotate, an antiferromagnetic film can be formed directly in contact with one of the magnetic films to apply anisotropy to the magnetic film in a predetermined direction based on exchange coupling.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system in which one of the magnetic films has means for applying a first magnetic anisotropy in a direction substantially perpendicular to a recording medium and the other magnetic film has means for applying a second magnetic anisotropy that is smaller in magnitude than the first magnetic anisotropy in a direction parallel to the medium in a plane of the other magnetic film. As means for applying the second magnetic anisotropy smaller in magnitude than the first magnetic anisotropy, there is a method of utilizing shape or uniaxial anisotropy of the other magnetic film, of disposing a suitable shunt or soft magnetic film adjacent to the other magnetic film, or of directly contacting the other magnetic film with another bias film.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system wherein one of the magnetic films has a first magnetic anisotropy directed in a direction substantially perpendicular to a recording medium and the other magnetic film has a second magnetic anisotropy that is smaller in magnitude than the first magnetic anisotropy and that is directed in a direction parallel to the recording medium in a plane of the other magnetic film.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system wherein an angle between magnetization directions of the magnetic films varies in response to a magnetic field from a recording medium, one of the magnetization directions is substantially fixed, and the other magnetization direction rotates in response to the magnetic field.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system wherein magnetic anisotropies applied to at least two magnetic films are different from each other in direction or magnitude.
In a magnetic storage/read system of the present invention, magnetic films each have shape anisotropy in a direction parallel to a recording medium in a plane of the magnetic films.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing thereon a signal with a predetermined track width and also comprises a magnetoresistive element having a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films and which has a pair of electrodes for applying a current to the structure for detecting a leakage magnetic field from the medium, wherein a height of the structure in a direction perpendicular to the medium is smaller than a length of the structure between the electrodes, and the length between the electrodes is smaller than a width of a track formed on the medium. Thus, this causes the shape anisotropy to effectively act on the reproduction part of the magnetoresistive element, whereby the output range of the magnetoresistive element can be compensated for and a tracking error in a reproduction mode wherein a signal from a desired track is mixed with a signal from an adjacent track can be prevented.
Further, in accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a magnetoresistive element for reading out a magnetic field from a magnetization pattern written on a recording medium with a predetermined track width, wherein a height d (xcexcm) of the element in a direction perpendicular to the medium and a track density Tr (tracks/inch) on the medium satisfy a relationship of dxe2x89xa6(12.5xc3x97103)/Tr. This relationship was obtained by experimentally making a magnetic storage/read system based on the present invention and testing the system with respect to its write and read ability.
In particular, with the magnetic storage/read system having the aforementioned magnetoresistive element mounted therein as a reproduction or read part, by prescribing a height of the element perpendicular to a face of the element opposed to the recording medium, i.e., the so-called MR height d, a good reproduction characteristic can be obtained.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon, a write head for recording a signal on the medium, and a read head for reproducing the signal from the medium, wherein the read head comprises a magnetoresistive element having a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon, a write head for recording a signal on the medium and a read head having a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films for reproducing the signal from the medium, wherein a width of a slider face of the read head opposed to the medium in a track width direction is smaller than 0.8 times a width of a face of the write head opposed to the medium in the track width direction. This value of 0.8 was found by experimentally making a magnetic storage/read system based on the present invention and testing it with respect to its reproduction ability.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and a magnetoresistive element for detecting a leakage magnetic field from the medium, wherein the element has a resistance change ratio which changes by 4.0 to 8.5% in response to a change of xc2x110 Oe in the leakage magnetic field from the medium.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and a magnetoresistive element having a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films for detecting a leakage magnetic field from the medium, and wherein the element has a resistance change ratio which changes by 5.0 to 9.5% in response to a change of xc2x18 Oe in the leakage magnetic field from the medium. These values were obtained by experimentally making a magnetic storage/read system based on the present invention and testing it with respect to its reproducing output, and were indispensable for attaining a high recording density with the present element.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and a magnetoresistive element for detecting a leakage magnetic field from the medium, wherein the element has a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films, an angle between magnetization directions of the magnetic films varies with a leakage magnetic field from the medium, the magnetization direction of one of the magnetic films is substantially fixed, and the magnetization direction of the other magnetic film rotates.
In a magnetic storage/read system of the present invention, in addition to the above arrangement, anisotropy is applied to the other magnetic film in a direction perpendicular to the direction of the leakage magnetic field.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a substrate, an inductive type write head for recording a signal, and a magnetoresistive read head for reproducing the signal, wherein the read head has a sandwich structure in which a nonmagnetic conductive film is disposed between magnetic films, and the write head is formed between the substrate and the read head.
A magnetic storage/read system of the present invention comprises a magnetoresistive element which includes a film structure wherein magnetic films and nonmagnetic conductive films are alternately stacked and also includes electrical terminals such as electrodes for applying a current to the film structure, wherein magnetization inducing anisotropy is applied to some of the magnetic films in a direction parallel to the direction of a magnetic field to be sensed. A magnetoresistive effect dependent on the orientation of the magnetization takes place in the interfaces between the magnetic films and the nonmagnetic conductive films such that the electric resistance of the element varies with the angle between the magnetizations of the magnetic films separated by the nonmagnetic conductive films and the magnetoresistive effect does not depend on the direction of a current flowing in the film plane.
In a magnetic storage/read system of the present invention, in addition to the above arrangement, anisotropy is applied to the remaining magnetic films in a direction perpendicular to the direction of the magnetic field to be sensed.
In the magnetic storage/read system of the present invention, since the recording density is increased, a recording area unit on the recording medium can be made correspondingly small, short or narrow. This is attained by making small the reproduction part of the magnetic recording unit.
In the present invention, this can be attained by suppressing a reduction in the sensitivity of the very small magnetoresistive element caused by the shape anisotropy of the magnetic films. That is, this can be attained by making the magnetic films thin, because the magnitude of the shape anisotropy of the magnetic film is substantially proportional to the thickness of the film. Meanwhile, for the purpose of preventing an output reduction caused by surface scattering, a total thickness of the magnetoresistive film element must be between about 100 and 300 xc3x85. This is because, even when the thickness of the respective magnetic films separated by the nonmagnetic films, and in particular the thickness of a soft magnetic film located in the middle of the element, is set to be below 100 xc3x85, especially below 10-20 xc3x85, this results in no reduction of the output. This action results from the fact that the physical mechanism of the magnetoresistive effect is based on the interfaces between the magnetic films and the nonmagnetic films.
In accordance with another aspect of the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and a magnetoresistive element for detecting a leakage magnetic field from the medium, wherein the element has a sandwich structure in which a nonmagnetic conductive film and magnetic films are laminated, and the magnetic films of the magnetoresistive element have a shape that is short in a direction perpendicular to a face of the element opposed to the recording medium.
In the magnetic storage/read system of the present invention, the magnetic films have a rectangular shape that is long in a direction parallel to the face of the element opposed to the recording medium and that has an aspect ratio of 2 or more, and the width w of the magnetic films in the above parallel direction and the height d of the magnetic films in the perpendicular direction satisfy a relationship of dxe2x89xa6w.
That is, one example of means for inducing magnetization is to form the magnetic films of the magnetoresistive element into a rectangular shape which is long in the direction parallel to the face of the element opposed to the recording medium and to provide an aspect ratio of 2 or more to induce shape anisotropy of the magnetic films in this direction. For the purpose of further increasing the recording density of the magnetic recording unit, the height (the so-called MR height d) of the magnetic films in a direction perpendicular to a face of the magnetoresistive element opposed to the recording medium and a width w of the magnetic films in a direction perpendicular to the above direction should satisfy a relationship dxe2x89xa6w. This is because substantial shape anisotropy is often influenced by the shape of the electrodes.
In the magnetic storage/read system of the present invention, when the magnetic films are formed into a rectangular shape which is short in the direction perpendicular to the face of the element opposed to the recording medium and which has an aspect ratio of 2 or more and the height d of the magnetic films in this direction is set to be below 5 xcexcm, the magnetic storage/read system can effectively detect a magnetic field on the recording medium.
This is because the size or thickness of the magnetic films are prescribed in order to induce a suitable magnitude of shape anisotropy in the magnetic films of the magnetoresistive element serving as magnetization inducing means.
With the magnetic storage/read system to the present invention, a recording density can be increased to about 2-10 times that of the prior art, and in particular the reproducing performance of the magnetoresistive element of the reproduction part can be enhanced to about 1.5-20 times.
Technical features, which form the base of the present invention, will be explained in the following.
A first feature is that magnetic films in the magnetoresistive element are separated by a very thin nonmagnetic film.
More specifically, the magnetoresistive effect caused by a difference between magnetization directions of the separated magnetic films is utilized, the thickness of the magnetic films is made small due to their separation, whereby the generation of magnetic anisotropy based on the shape of the element as well as a reduction in the sensitivity of the element to the magnetic field can both be prevented. As a result, miniaturization of the magnetoresistive element can be realized without deterioration of its reproducing ability. Further, the nonmagnetic film is made to be sufficiently thin with respect to its resistivity so that electrons can travel between the magnetic films through the nonmagnetic film, whereby a spin-direction-dependent magnetoresistive effect can take place. In addition, the thickness and structure of the magnetic films are controlled to make zero the magnetic coupling between the magnetic films or make the coupling small when compared to a leakage magnetic field from the recording medium, whereby the sensitivity of the element can be made high.
A second feature is that the magnetization directions of the magnetic films separated by the nonmagnetic film are induced in a specific direction. That is, the magnetization of one of the films is induced strongly in a direction parallel to the incoming direction of the leakage magnetic field from the recording medium. This direction is parallel to a direction normal to the face of the magnetoresistive element opposed to the recording medium. As a result, the magnetization of one of the magnetic films in the magnetoresistive element can be fixed in the above direction, while the magnetization of the other magnetic film is allowed to rotate in response to the leakage magnetic field from the recording medium, which enables the output of the magnetoresistive element to be made stable.
A third feature is that the magnetization of the other magnetic film is induced weakly in a direction perpendicular to the incoming direction of the leakage medium field. The term xe2x80x98induced weaklyxe2x80x99 means that the inducing of the magnetization of the other magnetic film is weaker than the fixing of the magnetization of the one magnetic film.
As a result, the rotation of the magnetization of the other magnetic film can advantageously be promoted and thus sensitivity at high frequency can be enhanced and noise can be suppressed. Further, since the output at zero field is prescribed, the magnetic storage/read system can respond to both of positive and negative magnetic fields.
In accordance with the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and a magnetoresistive element for detecting a leakage magnetic field from the medium, wherein the magnetoresistive element outputs a signal which varies stepwise in response to a variation in the leakage magnetic field.
The term xe2x80x98stepwisexe2x80x99 means that the response characteristic of the magnetoresistive element to the leakage magnetic field is not triangular but rectangular, that is, the characteristic has a part which varies relatively steeply with the leakage magnetic field and a part which is constant, i.e. which does not vary with the leakage magnetic field.
In accordance with the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and an electric-magnetic transducer for detecting a leakage magnetic field from the medium, wherein the electric-magnetic transducer directly outputs a digital signal indicative of the detected leakage magnetic field.
That is, the above response characteristic of the magnetoresistive element of the present invention is utilized to detect a signal on the recording medium and simultaneously to non-linearly convert the signal into a rectangular wave signal and to directly output it as a digital signal. As a result, digitalizing circuits in a subsequent signal processing system can be omitted and the reproducing and signal processing speed can be made high with less error.
In accordance with the present invention, there is provided a magnetic storage/read system which comprises a recording medium for magnetically storing a signal thereon and a magnetoresistive element for detecting a leakage magnetic field from the medium, wherein the element includes means for applying a bias field higher than the leakage magnetic field and outputs a signal which varies continuously and linearly in response to a variation in the leakage magnetic field.
This utilizes the fact that the response characteristic of the magnetoresistive element of the present invention has a sharp step to linearly convert a signal on the recording medium and output it as an analog signal. The above sharp step of the response characteristic can be controlled by controlling the magnitude of anisotropy applied to the magnetic films, and therefore it is necessary to provide means for applying anisotropy that is nearly equal to or larger than the leakage magnetic field from the recording medium.
Further, the thickness of the magnetic films of the magnetoresistive element used in the present invention is set to be preferably between 5 and 1000 xc3x85 and more preferably below 100 xc3x85. This is to enable the magnetic films to have sufficient magnetization and to enable current in the magnetoresistive element to be fully effectively utilized in the magnetoresistive effect.
It is preferable that the nonmagnetic conductive film separating the magnetic films has a thickness of 5 to 1000 xc3x85. The thickness of the nonmagnetic conductive film is set so that electron conduction cannot be prevented and especially so that the antiferromagnetic or ferromagnetic coupling between the magnetic films can be kept sufficiently small. Specifically, it is preferable that a thickness t of the nonmagnetic conductive film expressed in xc3x85 and an electric resistivity xcfx81 of the nonmagnetic conductive film expressed in xcexcxcexa9xc2x7cm satisfy a relationship of t xc3x85xc3x97xcfx81xcexcxcexa9xc2x7cm less than 100 xc3x85xc2x7xcexcxcexa9xc2x7cm. When the nonmagnetic conductive film is made of Cu, for example, the film is made to have a thickness of desirably 10 to 30 xc3x85. As is well known in the art, the electric resistivity xcfx81 of Cu is approximately 1.7 xcexcxcexa9xc2x7cm. For a Cu film having a thickness t=30 xc3x85, 30 xc3x85 (t)xc3x971.7 xcexcxcexa9xc2x7cm (xcfx81)=51 xc3x85xc2x7xcexcxcexa9xc2x7cm, which is less than 100 xc3x85xc2x7xcexcxcexa9xc2x7cm. Thus, a Cu film having a thickness of 10 to 30 xc3x85 satisfies the relationship of t xc3x85xc3x97xcfx81xcexcxcexa9xc2x7cm less than 100 xc3x85xc2x7xcexcxcexa9xc2x7cm.
The magnetic films, which are preferably soft magnetic films, are made preferably of an alloy of 70-95 atomic % of Ni and 5-30 atomic % of Fe.
Further, the magnetic films are made preferably of the aforementioned Ni-Fe-series alloy with less than 5 atomic % of Co suitably added. Alternatively, the magnetic films are made desirably of an alloy thin film of a face centered cubic structure made of 30-85 atomic % of Co, 2-30 atomic % of Ni and 2-50 atomic % of Fe. In this case, a good layered structure can be formed, providing excellent soft magnetic characteristics and producing a large magnetoresistive effect.
The nonmagnetic conductive film is made preferably of at least one of Au, Ag and Cu. The nonmagnetic conductive film, when combined with the magnetic films, can produce a magnetoresistive effect, provide an excellent high conductivity and form part of a good layered structure.
In an example of an arrangement of the magnetoresistive element of the present invention, an NiO film, an NiFe film, a Cu film, an NiFe film, a Cu film, an NiFe film and an NiO film are sequentially stacked on a substrate to form a layered structure, and then a pair of electrodes are provided on the layered structure. Alternatively, an NiO film, a CoNiFe film, a Cu film, a CoNiFe film, a Cu film, a CoNiFe film and an NiO film are sequentially stacked on a substrate to form a layered structure, and then a pair of electrodes are provided on the layered structure.
As another example of an arrangement of the magnetoresistive element of the present invention, an NiO film, a CoNiFe film, a Cu film, an NiFe film, a Cu film, a CoNiFe film and an NiO film are sequentially stacked on a substrate to form a layered structure, and then a pair of electrodes are provided on the layered structure. With such an arrangement, reduction in the output caused by surface scattering can be highly effectively prevented and thus its effective output can be increased, and the thickness of the middle film can be made thin, whereby deterioration of the sensitivity of the element caused by the shape anisotropy of the magnetic films can be prevented without deterioration of the output.
In this way, in the magnetic storage/read system of the present invention, the magnetoresistive element can be used as a reproduction unit, whereby a high recording density can be realized, that is, the recording wavelength to be recorded on the recording medium can be made short. Further, recording can be realized with the narrow width of a recording track, a sufficiently high level of reproduced output can be obtained, and thus good recording can be attained.
With the magnetic storage/read system of the present invention, less noise and a high reproduced output can be realized and recording can be carried out with an especially high recording density. This is because the magnetoresistive element of the invention can exhibit a change in a resistance change ratio as large as 4-10% in response to a change in the leakage magnetic field as small as xc2x110 Oe or less.
More specifically, in the magnetoresistive element used in the magnetic storage/read system of the present invention, a first magnetic film whose magnetization is induced strongly in a direction perpendicular to a face of the element opposed to the recording medium and a second magnetic film whose magnetization is induced weakly in a direction perpendicular to the above perpendicular direction are positioned opposing each other with a nonmagnetic film disposed therebetween.
This magnetoresistive element is disposed close to the recording medium to detect a leakage magnetic field reaching from the recording medium to the element in the form of a change in the electric resistance of the multilayer film. That is, in response to the magnetic field, the magnetization of the second magnetic film is rotated while the magnetization of the first magnetic film remains substantially fixed. For this reason, the angle between the magnetizations of the first and second magnetic films can vary stably with a variation in the leakage magnetic field and an output signal can be obtained based on the magnetoresistive effect.
Electrons travel between the two magnetic films through the nonmagnetic film and the relative difference between the magnetization orientations of the magnetic films causes the scattering probability to vary depending on the electron spin orientation, whereby a large magnetoresistive effect takes place. This effect does not depend on the direction of a current flowing in the film and the entire magnetization orientation.
The multilayered film of the magnetoresistive element is formed to have a field sensing width as small as less than 5 xcexcm and the height of the layered film is made to be about 1 xcexcm, so that the element can effectively detect a leakage magnetic field from the recording medium with a good sensitivity and can perform its reproducing operation with an especially high recording density.
The magnetic anisotropy based on the shape of the magnetic film can be approximately expressed by the following equation.
Hk=2/xcfx80xc3x97{tanxe2x88x921(tL/dr)xe2x88x92tanxe2x88x921(td/Lr)}xc3x97Bsxc3x97104
In the above equation, Hk (in oersteds) denotes the intensity of a shape anisotropy field acting on a rectangular magnetic material having a thickness t, a height d and a length L and tending to point in the longitudinal direction of the rectangular magnetic material, Bs denotes the saturation flux density of the magnetic material (in teslas), and r={square root over ( )}(t2+L2+d2). If t less than  less than L, d and dxe2x89xa6L/2 and Bs=1.0 using the value of Permalloy or Sendust as a typical magnetic material, then the shape anisotropy field becomes as follows.
Hkxcx9c0.4t(xc3x85)/d(xcexcm)
When a unidirectional anisotropic field of the bias film is between 100 and 200 Oe in the present invention, it is desirable that the shape anisotropy field Hk is from 0.4 to 100 (exclusive) Oe, that is, larger than the coercivity of the soft magnetic material and smaller than the bias field. With the magnetoresistive element of the present invention, even when the thickness of the ferromagnetic film is made as thin as 10-50 xc3x85, this does not cause reduction of the output. Accordingly, by the above equation, even in the case of a 1 xcexcm-high magnetoresistive element, the shape anisotropy field is as small as 4-20 Oe and thus the sensitivity of the magnetoresistive element will not be deteriorated. In the operation of the present invention, further, the above shape anisotropy field causes the magnetization direction of the magnetoresistive element to be prescribed. This effect is strengthened as the width of the magnetoresistive element in the track width direction is decreased. That is, sensitivity reduction when the size of the magnetoresistive element is made small can be minimized and further the operation of the magnetoresistive element is rather compensated for.
As already explained above, there can be realized a small-sized magnetoresistive head which produces a large output with good reproducibility with respect to a small external magnetic field. In the present invention, further, even when the above reproduction or read head is mounted on a magnetic disk device, its recording density can be increased by prescribing the recording wavelength, recording width, etc. From the comparison with a magnetic head using a prior art magnetoresistive element, the effects of the present invention can be evaluated. When the present invention is compared with the prior art magnetoresistive element using a Permalloy film of 200 xc3x85 thick with use of the above equation for evaluation of the shape anisotropy field, the read head of the present invention can have a shape anisotropy field about 10 times smaller. Assuming that the characteristics of the recording medium and the head-disk spacing characteristic in the magnetic disk device are the same in the present invention and in the prior art, then the reproducible recording wavelength in the present invention is only ⅓ of that in the prior art. Meanwhile, the output in the present invention is about 3 times that in the prior art because it is proportional to the resistance change ratio. When this is converted to track width, the present invention can generate the same output as the prior art with a track width of only about ⅓ of that in the prior art. From the above consideration, it will be seen that the recording density of the magnetic storage/read system of the present invention can be increased to about 10 times that of the prior art. More concretely, there can be provided a magnetic storage/read system which has a recording wavelength of 0.1 to 0.3 xcexcm, a track width of 0.2 to 4 xcexcm or a surface recording density of 0.5 to 30 Gb/in2.